When electronic devices (microprocessors, for example) are used in systems requiring fail safe capabilities, the designer must take special precautions to ensure that fail safe qualities are maintained, i.e., ensuring that failures are not unsafe. Electronic devices are different than vital relays in that such devices can fail in either one of the two states allowed for the device and accordingly, neither state can be faithfully assigned as more or less restricted. One of the techniques designers use to observe the vital characteristics in electronic circuitry is dynamic cycle checking; in this technique the permissive state is a controlled cycling of the device between its two different states, a steady condition in either state is restrictive. Examples of dynamic cycle checking are shown in my prior U.S. Pat. Nos. 3,995,173 and 4,090,173 entitled respectively, "SOLID STATE FAIL SAFE LOGIC SYSTEM" and "VITAL DIGITAL COMMUNICATION SYSTEM", both assigned to the assignee of this application.
When the input to an electronic system is the condition of a contact of a vital relay, special precautions must be taken to ensure that the state of the contact is dynamically cycle checked. This can be simply effected by feeding a dynamic signal over the contact of the vital relay, and in this regard see my above-referenced U.S. Pat. No. 3,995,173. This approach is not available if the contact is located more than a few feet away from the electronic system since the presence of a relatively long line coupling a dynamic signal to an electronic system makes the system unreliable (or even unsafe) by reason of noise or unwanted coupling to/from other circuits.